Field of the invention
The present invention relates to an electronically controlled device of a sewing machine for forming blind stitches, which may vary in dependence upon the thickness of a fabric to be sewn.
It is generally known that a blind stitch pattern has a width (W) between a left needle dropping position (L) and a right needle dropping position (R) as shown in FIG. 7. Further, it is generally known that the left needle dropping position (L) is a reference along which is guided an end part of the upper fold of a fabric to be sewn as shown in FIG. 8.
In FIG. 9, a basic line of a needle position is designated by an "X", which corresponds to one of the L, M, and R needle positions as is generally known. If the basic line X is set to the left needle dropping position L as shown in FIG. 9(a), the left needle dropping position (L) is fixed, and the right needle dropping position (R) is variable as shown by R.sub.1, R.sub.2, R.sub.3 relative to the left needle dropping position (L).
If the basic line is set to the right needle dropping position R as shown in FIG. 9(b), the right needle dropping position (R) is fixed, and the left needle dropping position (L) is variable as shown by L.sub.1, L.sub.2, L.sub.3 relative to the right needle dropping position R.
If the basic line is set to the middle needle dropping position (M) as shown in FIG. 9(c), the left and right needle dropping positions L, R are varied at the same ratio with respect to the needle position M.
The fabric F is folded as shown in FIGS. 10(a) to (c) to provide therein an upper fold F, and a lower fold F.sub.2 to be blind-stitched. The edge of the upper fold F.sub.1 is set laterally to a guide G of a blind stitching presser foot 17 as shown in FIGS. 8, 13, 16 and 17, in which the presser foot 17 is pressed against the upper and lower folds F.sub.1, F.sub.2 of the fabric F and the guide G is pressed against the lower fold F.sub.2. The guide G may be fixed to the presser foot 7 as shown in FIG. 13 or may be adjustable laterally of the presser foot 17 as shown in FIGS. 16 and 17.
It is then required to set the left needle dropping position L to an appropriate position on the fabric F where a machine needle 6 penetrates the upper and lower folds F.sub.1, F.sub.2 of the fabric F as shown in FIG. 8. More precisely the appropriate position is represented by (a) in FIG. 11, the position being slightly spaced away inward from the edge of the upper fold F, so that the formed stitches S may not be conspicuous as shown in FIG. 12(a) when the uppermost part of the fabric F is unfolded. The stitches S in FIG. 12(a) are shown in contrast to the conspicious stitches S.sub.1 which are undesirous and may be formed when the left needle dropping position L is set to the position (b) on the fabric F in FIG. 11 where the needle is spaced far away inward from the edge of the upper fold F.sub.1, compared with the needle position (a). Moveover such conspicuous stitches S.sub.1 will cause shrinkage of the fabric when the latter is unfolded. On the other hand, if the left needle dropping position L is set to a position (c) in FIG. 11 where the needle is spaced away outward from the edge of the uper fold F.sub.1, the blind stitches are not formed on the upper fold F.sub.1 and result in failure of the blind stitching function as shown in FIG. 12(c).
With respect to forming of the blind stitches, there have been generally three types as follows:
(1) A blind stitching presser foot having a guide fixed thereto is employed in combination with a sewing machine storing a blind stitch pattern of a predetermined amplitude;
(2) A blind stitching presser foot having a guide fixed thereto is employed in combination with a sewing machine storing a blind stitch pattern, the amplitude and basic line of which may be varied; and
(3) A blind stitching presser foot having a guide provided as being adjustable laterally thereof is employed in combination with a sewing machine storing a blind stitch pattern, at least the amplitude of which may be varied.
In case of item (1), the blind stitching presser foot 17 having the guide G fixed thereto as shown in FIG. 13 is employed in combination with the sewing machine as mentioned in the item. In this case, the needle positions are constant on both sides of the guide G. However because of the possible divergences in precision of parts in each of the sewing machines, it is generally designed that the guide G be mounted on the presser foot 17 such that the needle may drop at the left position L where the needle is sufficiently spaced away inward from the edge of the upper fold F.sub.1 to secure the correct formation of blind stitches. More precisely, the dimension (d) in FIG. 8 is amplified more than normally required, and this often causes the conspicuous (large) stitches S.sub.1 which are undesired.
In case of item (2), the blind stitches may be produced in the manner as shown in FIG. 9(a), FIG. 9(b) or FIG. 9(c), in which the amplitude may be varied with respect to the basic line L, R or M as mentioned thereinbefore. In case of FIG. 9(a), the right needle dropping position R may be variably set from the fixed left needle dropping position L. However as the distance (d) in FIG. 8 is predetermined by the guide G with an amplified dimension for the reason as mentioned above, this may cause such conspicuous stitches S.sub.1. In case of FIG. 9(b), the left needle dropping position L may be variably set from the fixed right needle dropping position R. However as the distance (d) in FIG. 8 is predetermined by the guide G with an amplified dimension, this may cause the lower fold F.sub.2 of a considerable size of dimension E to the left loose as shown in FIGS. 14 and 15. In case of FIG. 9(c), if one of the left and right needle dropping positions L, R is varied, and therefore the defects as mentioned in item 1 and in case of FIG. 9(b) may result.
In case of item (3), the guide G may be adjusted laterally of the presser foot 17 as shown in FIGS. 16 and 17 to variably determine the distance (d) of the blind stitch in FIG. 8. However the mechanical play of the presser foot structure will fail to stabilize the guide G in the set position relative to the presser foot 17 resulting in divergences of the blind stitches formed on the fabric.
For solving the above mentioned problems, the present invention incorporates a stitching pattern selecting device, a blind stitch width setting device and a blind stitch depth setting device which are independently set by operation of said selecting device in a sewing machine which stores stitch control signals in an electronic memory composed by a microcomputer, and connects said setting devices with the electronic memory.
When the blind stitch pattern is selected, the blind stitch width setting device and the blind stitch depth setting device are made operative. The left needle dropping position is set by the blind stitch depth setting device, and the right needle dropping position or the whole amplitude is set by the blind stitch width setting device. The set conditions are stored in the memory in a form of data for forming the blind stitch pattern.